Magnetic bearings have several potential advantages over, the more conventional, rolling element bearings. These advantages are obtained from the fact that in a magnetic bearing (unlike a rolling element bearing) there need be no contact between moving elements. Eliminating contact eliminates friction which in turn eliminates the need for lubrication. Friction also causes energy losses, wear and noise. Thus a magnetic bearing, potentially, is more efficient (in terms of energy loss), has a longer life-time, and is quieter than a rolling element bearing. These are the principal advantages to be gained from magnetic bearings.
Magnetic bearings however suffer from a major disadvantage when compared to rolling element bearings. That disadvantage is the stiffness of the bearing. Stiffness refers to the behaviour of the bearing when subjected to varying loads and specifically how much movement or "play" is induced in the bearing by a varying load. In a rolling element bearing where there is metal to metal contact the stiffness is very high as play is resisted by the metal itself. In a magnetic bearing however the resistive force is a function of the density of the magnetic field. When the load on a magnetic bearing is increased therefore the bearing will move until the magnetic field density has increased sufficiently. If the magnetic field is only changing slowly the bearing will have to move a large distance for this to happen and the bearing will have a low stiffness.
It is known to vary the magnetic field density by varying the current in the coils of an electromagnetic bearing, however this is not possible with magnetic bearings which use superconductors.